Deep electromagnetic investigations after April 6, 2009 earthquake to characterize the L'Aquila Basin

In consequence of the strong earthquake occurred in the Abruzzo region (Central Italy) on April 6, 2009 with magnitude Mw=6.3, two different type of geophysical investigation, magnetotelluric (MT) and Deep Electrical Resistivity Tomography (DERT), have been carried out across the Aterno Valley. The purpose of this type of deep geophysical exploration has been to characterize a wide portion of the subsurface defying the thickness and the geometric relationships between the quaternary deposits and the deeper mesocenozoic bedrock of the L'Aquila basin and the electrical structure at seismogenic depth. The MT surveys were carried out along a profile about 20 km long crossing the villages of Bagno (SW) and Paganica (NE) and placed in the epicentral zones of the aftershocks which is an area (30 km long and 10 km large) parallel to Apennine chain and oriented in NW-SE direction. The MT soundings were performed by MT24-LF systems (Schlumberger/EMI) installed in 15 different locations along the profile with a minimum site separations of 1 km. The frequency of data recording was set to 6.25 Hz for at least 24 hours and during night-time we launched high frequency sampling acquisition events (500 Hz) for at least 2 hour. Time series data were analyzed using the processing based on Egbert's robust code to compute estimates of the MT transfer function in the period range 0.00931-238.313 s. Dimensionality and directionality analysis have been performed before the application of an inversion 2D routine to obtain the resistivity distribution of subsoil, as function of depth. Almost coincident with MT profile, a DERT with "dipole-dipole" array configuration has been carried out along a profile of 8 km between Bagno and Paganica villages, using 21 stations with an electrode spacing of 400 m and a maximum distance between current and potential probes 7-8 times the basic spacing. In this way, the electrode array geometry allows us to obtain an exploration depth of about 900 m. The acquisition system consists of a transmitting station, which injects the current (mA) into the ground and a new multichannel receiver device, which record the generated voltage signals (mV). Four remote multi-channel data loggers connected with radio communication system to a personal computer compose the receiving system. Therefore, at the same time 8 voltage recording from 5 to 20 min were acquired for each current injection, obtaining 116 voltage recordings, related to different positions of the electrodes along the profile. The second step consisted in the extraction of useful signal from voltage recordings, using advanced statistical tools for removing the cultural noise, and to calculate the apparent resistivity values. Finally, we used the inversion algorithm RES2Dinv to determine a 2D resistivity model. The results of these deep geophysical investigations can give some essential elements to the knowledge of the geological and structural setting of the area to better understand the framework in which the L'Aquila earthquake took place.